1. Field of the Invention
The present invention relates to a method and a device for co-operative radio direction-finding in transmission for signals comprising a reference signal.
The invention can be applied especially to the case of signals comprising a synchronization signal or reference signal, consisting of a sequence of known symbols defined by the transmission standard used within the transmission system.
For example, the GSM (Global System for Mobile communications), consisting of several reference signals, has this characteristic. In such a context, a reception system such as a base station may perform a selective direction-finding operation and associate a direction of arrival with a synchronization sequence. The techniques of selective direction-finding can be used to isolate one transmitter from the others on the basis of knowledge of its reference signal. It is thus possible to carry out a selective direction-finding operation for the transmitter, thus reducing the risk of ambiguity.
2. Description of the Prior Art
The patent FR 2 764 074 describes a method for carrying out a direction-finding operation in the case of single-user operation and for linear modulations or waveforms that may be approximated in terms of linear modulations. In this patent, linearly modulated digital signals se(t) are sent. The symbols a1, a2, a3, etc., associated with the signal, are transmitted at the period Ts and then filtered by a FIR (Finite Impulse Response) filter with a response h(t).
Thus, the following is the relationship between the transmitted signal se(t) and the symbols an:                                           s            e                    ⁡                      (            t            )                          =                              ∑            n                    ⁢                                    h              ⁡                              (                                  t                  -                                      nT                    s                                                  )                                      ⁢                          a              n                                                          (        1        )            
where h(t) corresponds to the shaping filter.
FIG. 1 gives a diagrammatic view of the principle of the method.
The example given in this patent relates chiefly to the GSM standard in which the signal sent is a GMSK (Gaussian Minimum Shift Keying) phase modulation such that:                                           s            e                    ⁡                      (            t            )                          =                  exp          ⁢                      {                          xe2x80x83                        ⁢                          j              ⁢                                                ∑                  n                                ⁢                                                      Φ                    ⁡                                          (                                              t                        -                                                  nT                          s                                                                    )                                                        ⁢                                      a                    n                                                                        }                                              (        2        )            
where an corresponds to a symbol sent at a rate of Ts samples per symbol.
In order that the GMSK standard may form part of the linear modulations, the authors of this patent have made the following approximation:                                                         s              e                        ⁡                          (              t              )                                ≈                                    ∑              n                        ⁢                                                            C                  0                                ⁡                                  (                                      t                    -                                          nT                      s                                                        )                                            ⁢                              j                n                            ⁢                              b                n                            ⁢                              xe2x80x83                            ⁢              with              ⁢                              xe2x80x83                            ⁢                              b                n                                                    =                              ∏                          k              =                              -                ∞                                      n                    ⁢                      a            k                                              (        3        )            
The linearization is obtained, for example, by applying the method described in reference of Pierre Andre Laurent, xe2x80x9cExact and approximate construction of digital phase modulations by superposition of modulated pulse (AMP)xe2x80x9d in IEEE Trans. on Communications Vol 34 (1986) pp 150-160.
In the sequence of symbols {an}, there is a sequence of symbols {sk} such that ap+kxe2x88x921={sk} for 1xe2x89xa6kxe2x89xa6K. The sequence of symbols {sk} is defined by the transmission standard and is called a learning sequence. In particular, the GSM standard comprises eight different sequences formed by K=26 symbols.
The signal se(t) travels in transit towards the nth antenna of a reception system through an FIR linear filter propagation channel C(t). Under these conditions, there is a relationship of linear filtering between the comb of symbols s(t) (such that se(t)=h(t)*s(t) where  less than  less than * greater than  greater than  designates the convolution product) and the sensor signals xn(t), illustrated in FIG. 2.
Given that C(t)=[C1(t) . . . CN(t)]T, the relationship between the transmitted symbols an and the received signal x(t) is expressed in the form:                                           x            _                    ⁡                      (            t            )                          =                                                            C                ⁡                                  (                  t                  )                                            *                        ⁢                                          s                e                            ⁡                              (                t                )                                              =                                                                      (                                                            C                      *                                        ⁢                                          h                      ⁡                                              (                        t                        )                                                                              )                                *                            ⁢                              s                ⁡                                  (                  t                  )                                                      =                                                            ∑                  n                                ⁢                                                      G                    ⁡                                          (                                              t                        -                                                  nT                          s                                                                    )                                                        ⁢                                      a                    n                                                              +                                                                    b                    _                                    0                                ⁡                                  (                  t                  )                                                                                        (        4        )            
where
for t=nTs, s(t)=an and for txe2x89xa0nTs, s(t)=0.
G(t)=C(t)*h(t) is a Nxc3x971-sized vector and b(t) is the vector formed by the background noise of the N receivers.
The method disclosed in the patent FR 2 764 074 is subdivided into three steps:
1st step: Digitization of the signal received x(t) at the sampling period Te. Since the input of the system is formed by symbols {an}, Te must be chosen so that Ts=P Te where P is an integer. Thus, if we write Gpxe2x88x92nP=G((pxe2x88x92nP)Te) the relationship (4) becomes:                                           x            _                    ⁡                      (                          pT              e                        )                          =                                            ∑              n                        ⁢                                          G                                  p                  -                  nP                                            ⁢                              a                n                                              +                                                    b                _                            0                        ⁡                          (                              p                ⁢                                  xe2x80x83                                ⁢                                  T                  e                                            )                                                          (        5        )            
In the description of the patent, the authors write P=2. The fact of choosing the sampling period Te as a function of the symbol period Ts is a is heavy constraint. Generally, the equipment cannot be used to obtain Ts=P Te (P integer) directly. The sampling is done at Txe2x80x2e and, thereafter, to have a sampling at Te, it is necessary to carry out digital processing operations which resample the signal at Te. In the GSM standard, the signals sent verify the relationship 1/Ts=270 kHz. To meet the conditions of the Shannon theorem at reception, the system receives them at 1/Te=500 kHz and Ts/Te=500/270 is in fact a non-integer.
2nd step: Synchronization procedure: the goal is to determine the instant t0=pTe of appearance of the learning sequence {sk} such that ap+kxe2x88x921={sk} for 1xe2x89xa6kxe2x89xa6K:                                           x            _                    ⁡                      (                                          (                                  k                  +                  p                                )                            ⁢                              T                e                                      )                          =                                            ∑              n                        ⁢                                          G                                  k                  -                  nP                                            ⁢                              s                n                                              +                                                    b                _                            0                        ⁡                          (                                                (                                      k                    +                    p                                    )                                ⁢                                  T                  e                                            )                                                          (        6        )            
To this end, it is considered that the symbol comb s(n Te) is the input signal and it is sought to maximize the following synchronization criterion in terms of xcfx84:                                           Cri            ⁡                          (              τ              )                                =                                                                                          r                    ^                                    xs                                ⁡                                  (                  τ                  )                                            H                        ⁢                                          R                xx                                  -                  1                                            ⁡                              (                τ                )                                      ⁢                                                            r                  ^                                xs                            ⁡                              (                τ                )                                                    ⁢                  
                ⁢                              With            ⁢                          xe2x80x83                        ⁢                                                            r                  ^                                xs                            ⁡                              (                τ                )                                              =                                    1              KP                        ⁢                                          ∑                                  k                  =                  1                                KP                            ⁢                                                                    x                    _                                    ⁡                                      (                                                                  k                        ⁢                                                  xe2x80x83                                                ⁢                                                  T                          e                                                                    +                      τ                                        )                                                  ⁢                                                      s                    ⁡                                          (                                              k                        ⁢                                                  xe2x80x83                                                ⁢                                                  T                          e                                                                    )                                                        *                                ⁢                                  xe2x80x83                                ⁢                and                                                    ⁢                  
                ⁢                                            R              xx                        ⁡                          (              τ              )                                =                                    1              KP                        ⁢                                          ∑                                  k                  =                  1                                KP                            ⁢                                                                    x                    _                                    ⁡                                      (                                                                  k                        ⁢                                                  xe2x80x83                                                ⁢                                                  T                          e                                                                    +                      τ                                        )                                                  ⁢                                                                            x                      _                                        ⁡                                          (                                                                        k                          ⁢                                                      xe2x80x83                                                    ⁢                                                      T                            e                                                                          +                        τ                                            )                                                        H                                                                                        (        7        )            
K: number of symbols of the learning sequence
and s(n Te)=skP for n=kP and s(n Te)=0 for nxe2x89xa0kP and where xH designates the transposition and conjugate of x.
3rd step: The signals x(t+tk) and the symbol comb filters s(n Te) associated with a source are used in order to isolate this source and carry out a direction-finding operation on a single source at the instant t0=p Te. With the use of the received signal x(t+tk) and a single symbol comb filter s(n Te), the method is said to be a single-user method.
While the prior art method performs well, it nevertheless has certain drawbacks. It can be applied to waveforms that can be linearized or that can be approximated in such a form, and in single-user contexts.
The method according to the invention relies on a novel approach that consists especially in exploiting the signal or reference sequence containing symbols proper to a standard used or to a given format for the signal, the received signal and the knowledge of the instance of arrival of the different reference sequences, to determine the coordinates of a source from a measurement of its incidence, this operation being commonly known as direction-finding.
In the description, the terms xe2x80x9ctransmittersxe2x80x9d, xe2x80x9cusersxe2x80x9d or xe2x80x9cuseful transmittersxe2x80x9d designate one and the same object.
The invention relates to a method of co-operative radio direction-finding in transmission in a system of radio direction-finding for a radiocommunications system comprising one or more transmission sources or transmitters, the transmitted signal comprising a reference sequence formed by a sequence of symbols {sk}, the receiver comprising at least one array of several sensors coupled to a radio direction-finder. The invention is characterized in that it comprises at least the following steps:
a) determining the reference signal or signals di(t) associated with a transmitter m from the symbols and from the modulation defined by the format of the signals,
b) defining the instant of arrival tk of one or more reference signals,
c) from the signal x(t) received at the sensors, the instants of arrival tk and the reference signal or signals di(t) derived from the step a), determining the value or values of incidence of the signals for a given transmitter.
The step a) is reiterated as many times as there are transmitters M0 in the system.
It may comprise a step for the separation of the transmitters M0 through the estimation of the transfer matrix of the channel Hu for the transmission of all the useful signals and by the extraction, from this matrix, of the transfer matrices Hmu of the channel associated with a useful transmitter m in order to determine the corresponding observation vector xm(t).
The step c) is carried out by means of a direction-finding operation by whitening, implemented on the observation vectors xm(t) or by projection.
The step a) may be performed on a duration xcex4txe2x80x2 greater than xcex4t, where xcex4t is the basic duration of the reference signal.
A spatial filter Wm can be determined from the reference signal dm(t) of a transmitter m in order to deduce the signal sm (t) coming from the transmitter before applying a direction-finding step.
The method can be applied especially in radio communications systems for GSM or UMTS type signals.
The invention also relates to a radio direction-finding device comprising at least means adapted to implementing the steps of the above-mentioned method.
The object of the invention has the following advantages in particular:
the possibility of performing a direction-finding operation in a context that is wider (namely a single-user or multiple-user context and a single-path or multipath context) than the one offered by the prior art method.
through the choice of the working time span or observation time span, the possibility of improving the results of the direction-finding,
through the modification, for example the lengthening, of the duration of the reference signals, the possibility of improving the results of the direction-finding operation,
the possibility of localizing mobiles to make directional transmission toward each mobile, on the basis of the knowledge of the direction of arrival of signals from the mobiles whose direction has been located,
the possibility of localizing mobiles or base stations in a context of spectrum control,
the reduction of the transmission power at constant range or the increasing of the transmission range at constant power, and consequently the reduction of interference with respect to the other cells,
the method is applied for different waveforms, without specific processing,
through the use of whitening or projection techniques that enable a more efficient rejection of interference, the possibility of improving the direction-finding results.